Understanding the cellular responses to tumor necrosis factor (TNF) has remained an elusive goal. Although every nucleated cell type examined to date displays TNF-receptors (TNF-Rs), ligand binding does not predict biologic responsiveness. Upon isolating and cloning the genes for human TNF-Rs, it was clear that these ligand binding peptides lacked consensus sequences typical of known kinases or substrates for kinases, thereby strongly suggesting the co-association of accessory molecules for the formation of a biologically active TNF-R complex. Preliminary evidence has shown that the lymphocyte TNF-R can functionally associate with another cell surface protein. This protein, designated Fas, is predicted to function as a signal transduction molecule. In addition to its potential role in transmembrane signalling, Fas can regulate additional cellular processes leading to differential apoptotic cell death in activated lymphocytes. To date, little is known about the molecular nature and function. of Fas. The research plan proposed in this application is designed to explore such parameters. Specifically, this includes characterizing the molecular nature of the lymphocyte Fas molecule, comparing it to that of myeloid and hematopoietic tumor cell lines, determining its physical association with the TNF-R, and elucidating its role in transmembrane signalling. Tbe knowledge gained from these studies will extend our understanding of the obligate requirement for TNF in cytotoxic lymphocyte development and may provide the therapeutic basis for exploitation of Fas-triggered apoptotic cell death in autoimmune disease, graft rejection and/or malignancy.